Town boss Chris Wilder has continued his summer reshaping with the capture of the 29-year-old who joins from Port Vale for the entire 2014/15 campaign.
Cardiff-born Mohamed spent the majority of last season in League Two with loan spells at AFC Wimbledon and Bristol Rovers. He played against Northampton in a goalless draw at Sixfields at the beginning of March.
Assistant manager Alan Knill, meanwhile, has confirmed that experienced striker Jon Parkin is on the club's list of potential signings after his release by Fleetwood.